Leydig cells are regulated within a hypothalamicpituitary-testicular axis. The hypothalamus produces GnRH, which stimulates pituitary gonadotropes to secrete LH and FSH. Testosterone, DHT, and estradiol negatively feed back at the pituitary and hypothalamus and inhibit LH more than FSH secretion. Inhibin from the Sertoli cells selectively inhibits FSH.
10.
Testosterone, DHT, and estradiol have numerous actions on the male reproductive tract, external genitalia, and male secondary sex characteristics, as well as on other organ systems (e.g., blood cell production, lipoprotein production, bone maturation).
11.
The male tract includes tubal structures (epididymis, ductus deferens, and male urethra), accessory sex glands (seminal vesicles, prostate), and the penis. The seminal vesicles and the prostate produce most of the ejaculate, which nourishes, buffers, and protects sperm.
12.
Penile erection involves a complex neurovascular response leading to engorgement of the erectile tissue within the penis base and shaft with blood.
13.
The follicle is the functional unit of the ovary. Follicles contain epithelial cells (granulosa and cumulus) and outer stromal cells (thecal). All these cells surround a primary oocyte that remains arrested in the first meiotic prophase until just before ovulation.
14.
Follicles develop from the smallest (primordial) to a large antral follicle over a period of months. The latter part of follicular development requires gonadotropins.
15.
The menstrual cycle refers to an approximately 28-day cycle that is driven by the following ovarian events: development of one large antral follicle to a preovulatory follicle (follicular phase), ovulation, and formation and death of a corpus luteum of menstruation (luteal phase).
16.
The follicular phase of the ovary corresponds to the menstrual and proliferative phases of the uterine endometrium. The luteal phase of the ovary corresponds to the secretory phase of the uterine endometrium.
17.
One dominant follicle is selected per menstrual cycle—usually the largest follicle with the most FSH receptors.
18.
High levels of estradiol occur around midcycle and exert positive feedback on gonadotropin secretion. This induces the LH (and a smaller FSH) surge. The midcycle gonadotropin surge induces (a) meiotic maturation of the primary oocyte so that it progresses to a secondary oocyte (with one polar body) arrested at metaphase of the second meiotic division, (b) breakdown of the ovarian and follicular wall so that the oocyte-cumulus complex is extruded (called ovulation), and (c) differentiation of the remaining follicular cells into a corpus luteum. The corpus luteum produces high levels of progesterone, estradiol, and inhibin.
19.
If pregnancy does not occur, the corpus luteum will die in 14 days. This constitutes the luteal phase of the menstrual cycle.
20.
The oviducts capture the ovulated cumulus-oocyte complex and transport it medially into the oviduct and toward the uterus. Estrogen promotes ciliation and transport; progesterone inhibits transport.
21.
The uterine mucosa, called the endometrium, is the normal site of embryonic implantation. The mucosa is increased in thickness in preparation for implantation and is sloughed away if no pregnancy occurs.
22.
During the mid to late follicular phase (days 6–14 of the menstrual cycle), the ovary produces estradiol, which induces all cells of the endometrium to proliferate (called the proliferative phase of the uterus).